In prior art cellular radio telephone systems, a geographical area is covered and served by breaking the area into a plurality of small zones or cells. A large land area can be covered by a plurality of connected cells areas. A mobile telephone switching office (MTSO) is provided for each area and is connected to a plurality of base stations, each covering a cell within the area.
FIG. 1 is a diagram illustrating a conventional cellular radio telephone system. Referring to FIG. 1, the user of cellular radio telephone 101 usually travels within an area 102 and is assigned an identification (ID) number for the user's cellular radio telephone which is registered in a corresponding MTSO 103 which covers the area. The area is called a home area and the user is called a home area user. If the user travels out of area 102 and enters another area, the new area is called a roam area and the user is called a roamer in the new area. User fees for calls from the cellular radio telephone via the MTSO covering the roam area are higher than there via the MTSO covering the home area. It should be noted that, when a call is made from a cellular radio telephone, the ID number for the cellular radio telephone is transmitted to the MTSO covering the area where the cellular radio telephone is located and the MTSO can therefore distinguish roamers and home area users by checking whether the transmitted ID number is registered in the MTSO. Accordingly, some conventional cellular radio telephones have a plurality of ID numbers so that each of the ID numbers, each one of which is registered to a different area.
FIG. 2 is a further diagram illustrating a conventional cellular radio telephone system. Referring to FIG. 2, if the user of cellular radio telephone 201 registers ID #1 to MTSO 203 covering area #1 and ID #2 to MTSO 204 covering area #2, either area #1 or area #2 could become the home area for the user by using the ID number registered in the MTSO covering the area when a call is made.
In a conventional cellular radio telephone as mentioned above, an ID number is manually selected by the user. In the situation shown in FIG. 2, when a user is within area #1 and makes a call, the user has to select ID #1 by operating a changeover switch provided in the cellular radio telephone to reduce speech charges. Otherwise, ID #2 may be sent to MTSO 203 covering area #1 which would result in higher speech charges because MTSO 203 identifies the call as being that from a roamer. Therefore, it is necessary for a user to pay attention to whether the proper ID number for the area is selected when the user makes a call. In some situations, however, a user does not exactly know which area he presently is in. Thus, the user cannot select the proper ID number.
Furthermore, another problem exists in receiving an incoming call. Once an ID number is selected by operating the changeover switch in a cellular radio telephone, the telephone receives only incoming calls having the selected ID number. Accordingly, when an ID number is selected, if a caller knows only another ID number, a call from the caller is never received by the cellular radio telephone even though the system broadcasts a paging signal accompanied with the other ID number and the cellular radio telephone is not busy.